Friction stir welding (FSW) is a method used to join metal workpieces. The method generally uses a cylindrical, shouldered tool with a profiled pin that is rotated at the joint line between two workpieces while being traversed along the joint line. The rotary motion of the tool generates frictional heat which serves to soften and plasticize the workpieces. This softened material, contributed by both workpieces, intermingles and is consolidated by the pin shoulder. As the pin moves laterally the frictional heating is reduced and the softened material hardens, creating a bond between the two workpieces. The best current understanding of the process is that no melting occurs and the weld is left in a fine-grained, hot worked condition with no entrapped oxides or gas porosity.
Stir rods used in conventional FSW are typically symmetrical cylinders having an enlarged fixed cap located on their upper side. The fixed cap used in conventional FSW does not engage a workpiece until the end of tool insertion, allowing a majority of the initially plasticized material to be expelled from the cavity before the cap creates a seal around the worksite. Current methods used in FSW do not teach or suggest methods of engaging a cap and a workpiece at the beginning of the process to retain the maximum amount of plasticized material in the weld zone.